user-env-compile plugin

So I've seen the discussion here https://github.com/progrium/dokku/issues/202 regarding access to environment variables in the build phase. I generally agree with the points raised, however there is no escaping the fact that sometimes it is a really handy option to have available. I've added a dokku version of Heroku's user-env-compile feature, and it's written as a plugin so you could opt-in or opt-out as you prefer.

Whaddayareckon?